per #15423 Remove deprecated features from akka-persistence

* remove channels
* remove View
* remove Processor
* collapse the complicated internal state management
  that was spread out between Processor, Eventsourced and Recovery
* remove Recovery trait, this caused some duplication between Eventsourced
  and PersistentView, but but the enhanced PersistentView will not be based
  on recovery infrastructure, and therefore PersistentView code will be replaced anyway
* remove PersistentBatch
* remove LoopMessage
* remove deleteMessages of individual messages
* remove Persistent, PersistentRepr and PersistentImpl are kept
* remove processorId
* update doc sample code
I think this is wrong: if the user just used `persistAsync` then we’ll want to unstash, otherwise too fresh messages will be processed. Please add another test that verifies that no message reorderings occur.

To make it really clear what I mean, I’d change to
~~~scala
if (resequenceableEventBatch.nonEmpty) flushBatch()

if (pendingStashingPersistInvocations > 0) changeState(persistingEvents)
else internalStash.unstash()
~~~